A programmable logic controller can be used for automating and controlling industrial installations. To this end, the programmable logic controller comprises a signal interface for inputting and outputting signals. The signals can comprise analogue or digital signals for controlling a field device, for example an actuator or a sensor. Depending on the field device to be controlled, the signals used for control can have different voltages and current values. In particular when controlling relays, for example by means of a relay output board, voltages of up to 250V and current values of up to 8 A can be achieved.
The programmable logic controller is often connected to the field device via a series terminal. To electrically connect the programmable logic controller to the series terminal a so-called front adapter can be used. The front adapter can be pluggable into the signal interface of the programmable logic controller and can comprise electrical connecting lines for the electrical connection of the signal interface of the programmable logic controller to electrical connectors of the series terminal. In the process, the front adapter can be adapted geometrically to the signal interface of the programmable logic controller. Such front adapters are often only suitable for comparatively low voltages of up to 24V due to electric-shock protection.